Field of the Invention
The present invention relates to a chuck table for use in a processing apparatus such as a laser processing apparatus or the like.
Description of the Related Art
According to a processing method that has started to be used in recent years for semiconductor wafers and optical device wafers wherein a light-emitting layer is formed on an epitaxial substrate made of sapphire, SiC, GaN, or the like, a laser beam which has a wavelength permeable to a wafer to be processed is applied to the wafer while being focused at a point within the wafer, forming a modified layer in the wafer, and then external forces are applied to the wafer to divide the wafer into individual device chips, with the modified layer acting as a fracture starting point (see, for example, Japanese Patent No. 3408805).
The above processing method is subject to the condition that anything (e.g., materials for forming various patterns that make up devices) likely to attenuate the energy of the laser beam should not exist in the wafer surface that is irradiated with the laser beam. Therefore, it has been the general practice to form a modified layer in a wafer having a plurality of devices on a front side thereof by applying a laser beam to the wafer from a back side thereof which is free of any devices.
In order for the back side of the wafer to be irradiated with the laser beam, the front side of the wafer on which the devices are formed needs to be held by a chuck table. There has been known a processing method for covering a holding surface of the chuck table with a sheet member so as to prevent the devices from being broken by direct contact with the holding surface of the chuck table (see Japanese Patent Laid-open No. 2013-229403).